It's not truly disaggregation, where you would treat it as two separate plans as you might be used to with 410(b) and 401(a)(4). Rather, what the new law says is that employees who have not met age 21/1 year of service can be disregarded when determining if a DC plan has satisfied the top heavy minimum. So it doesn't matter if there are any otherwise excludable key employees, you just ignore all of the under 21/under 1 year employees when determining who is entitled to a top heavy minimum.
Where it gets weird is with the safe harbor match. The IRS ruled (in rev. rul. 2004-13) that a plan which different eligibility for deferrals and safe harbor does not consist "solely" of deferrals and match meeting the safe harbor requirements, which is the rule to be treated as not top heavy under IRC 416(g)(4)(H). That clause wasn't affected by the new law. So presumably a plan with different eligibility for deferrals and match is still treated as top heavy, and subject to the top heavy minimum. The fact that they don't have to give the top heavy minimum to otherwise excludable employees doesn't change this, it just means that employees who are not otherwise excludable (over 21/1 year of service) will have to get the top heavy minimum. The top heavy minimum for these people could be satisfied by their safe harbor match contribution, or if they don't get any safe harbor (or enough safe harbor, because they didn't defer enough or not at all), then by an additional employer contribution.